1. Field of the Invention
This invention relates to a rotation angle measuring apparatus using a rotatable disk formed with a pattern for rotation angle detection.
2. Related Background Art
In a rotation angle measuring apparatus disclosed in Japanese Laid-Open Patent Application No. 55-67608, an index disk is provided concentrically with a rotatable disk having a radial grating of equiangular pitch on the circumference thereof so as to be opposed to the rotatable disk, and at rotation-symmetrical positions on the index disk, two index gratings of the same pitch as the radial grating of the rotatable disk are provided at opposite phases, and two light sources and two photoelectric conversion elements are provided with the two index gratings and the rotatable disk interposed therebetween. Two signal processing circuits are provided correspondingly to the two photoelectric conversion elements, respectively.
The output signals of the two photoelectric conversion elements are converted into pulses in the respective corresponding signal processing circuits, and the pulses are counted by counters. The count values of the two counters are averaged by an averaging circuit, and a display circuit converts the output value of the averaging circuit into a rotation angle and displays the rotation angle. When there is axial back-lash or eccentricity, the count values of the two counters include amounts of phase shift equal to each other in opposite directions, but by adding and averaging these count values by the averaging circuit, the influence of the eccentricity is eliminated and a right rotation angle can be measured.
Heretofore, a reference pattern has been formed only at an arbitrary location on the rotatable disk of a rotation angle measuring apparatus of this kind, and a corresponding reference pattern has been formed at a predetermined position on the index disk. The two counters are reset by the use of reference signals obtained from the photoelectric conversion elements when the reference pattern of the rotatable disk and the reference pattern of the index disk become opposed to each other, but when there is radial back-lash in the rotary shaft, the radial grating formed on the rotatable disk rotates eccentrically and thus, the reference signal includes an error. Further, the error is increased by the hysteresis or the like in the signal processing circuits, and when the amount of the error exceeds a prescribed value, the resetting of the counters may deviate by an amount corresponding to one pitch of the pattern, and this has caused the occurrence of a measurement error.